Metamaterials are a large class of engineered materials. Metamaterial properties are determined from the organization of the constituents. Current metamaterial research is focused on novel optical devices, for example, flat sub-wavelength resolution macroscopic lenses (see refs. 1-5) and transformational optics (see refs. 3, 6-8). Top-down lithographic techniques (see refs. 9-12) have, in general, been used to create nanostructured metamaterials. Typically the processes are complex, time consuming, expensive, producing primarily 2D fixed structures with limited particle resolution. Another strategy to generate nanostructured metamaterials is bottom-up or self-assembly, but this has proved challenging, both from a fundamental and production approach, over the last decade (see refs. 13, 14). If an efficient self-assembly process can be realized to organize nanometer size constituents into macroscopic homogenized materials, then practical metamaterial devices may become possible.
Other investigators have suspended metallic nanoparticles in fluids for the purpose of making films. Deng (see ref. 15) developed a method for making nanoparticle films at the air-fluid interface and dip-coated a substrate into the suspension to remove the nanoparticles from the interface. However, the nanospheres were not packed efficiently in any positional order, the size of domains transferred onto the substrate was only submicron in size, the nanospheres were not capable of being phase transferred, the mechanism for the nanosphere aggregating at the air-fluid interface was not presented and the film had to be transferred onto required substrates. Jaeger (see ref. 16) evaporated sessile droplets of gold nanospheres and ligands in organic suspension. They achieved macroscopic, self-assembled, monolayer domains of gold nanospheres at the air-liquid interface, but the nanoparticles were only gold nanospheres, could not be transferred to other substrates and could not be crosslinked together to form a film. Sastry (see ref. 17) observed thin films of gold nanospheres confined between a liquid-liquid interface translate up a glass vial via Marangoni flow (see ref. 18). Spain (see ref. 19) observed silver nanospheres suspended in excess ligand and two immiscible fluids translate up the walls of a vial using Marangoni flow. Both Sastry and Spain required a liquid-liquid interface, the nanoparticles were only nanospheres and the films were not high-density crosslinked monolayers. Sastry (see ref. 20) also crosslinked gold nanosphere films at the liquid-liquid interface with benzene and anthracene, but the crosslinking was not controllable and only micron-size domains were presented. Hoyle (see ref. 21) claimed trithiol functionalized gold nanoaggregates dispersed in thiol-ene films; however, the nanoparticle density was very dilute (0-1 wt %).
A need exists for techniques effective in creating a continuous, uniform, and tightly-packed monolayer of nanoparticles, particularly at larger scales.